The resolution of a lens-based imaging system is defined by the finite dimensions of a lens. The resolving power or the minimum separation between two points which can be resolved is limited by the diffraction that takes place in the imaging system (e.g. microscope) because of the wave nature of light. Moreover, it should be noted that the resolution limit arises from the assumption that the image of an object is being detected in the “Far-Field”, that is, at a distance which is much greater than the wavelength of light employed. For these reasons, where the optical microscopy is so limited in resolution to no better than one-half of the wavelength of light being used, such microscopy is termed “far-field” imaging or viewing. An alternative to conventional lens-based optical microscopy is the “near-field” or lens less technique which provides super resolution imaging and spectroscopy. The term “super resolution” refers to spatial resolution which exceeds the diffraction limitation caused by the wave nature of light, and signifies a resolution which is greater than one-half the wavelength of the light actually being used. Super resolution near-field imaging and near-field scanning optical microscopy (“NSOM”) is based on the fact that although light cannot be focused to a spot less than one-half the wavelength of light, it can be directed through a device or article which reduces the size of the light energy to dimensions smaller than one-half the wavelength of light.
Despite the many recent developments and innovations reported in the scientific literature, near-field imaging and near-field scanning optical microscopy remains a difficult and burdensome technique in practice. The aperture device for producing sub wavelength light energy yields only a single beam of light, requires careful positioning of the single beam of light in a carefully controlled fixed relationship to the surface of an object to be imaged, and provides images which are extremely limited in the size of the field of view as well as the quality of the image actually seen. Moreover, to employ the NSOM technique, a long period of carefully controlled raster scanning must be performed using only the single beam of light which must then travel repeatedly over large areas of sample to provide even a single image. Thus, present practical techniques of NSOM and near-field imaging as such are highly laborious, time consuming, technically stringent and demanding, and provide only single points of information which then must be carefully matched with many other individual points in order to obtain a comprehensive picture of the sample which then may be reviewed for micro analytical purposes.
U.S. Pat. No. 5,633,972 describes a super resolution imaging fiber for sub wavelength light energy generation and near-field optical microscopy. The imaging fiber comprises a unitary fiber optical array of fixed configuration and dimensions comprising typically from 1,000 to 100,000 optical fiber strands which terminate at one array end as tapered strand end faces limited in size to a range from about 2-1,000 nanometers in diameter. Overlying these tapered strand end faces is a thin opaque metal coating having a size-limited end aperture ranging from about 2 to less than about 1,000 nanometers in diameter. These size-limited end apertures collect the evanescent light scattered from the surface of the object and transfer the energy to an output detector. That way a generation of a plurality of discrete sub wavelength light beams is concurrently generated.
The basis of operation of a second type of NSOM, the conventional scanning tunneling optical microscope (STOM), also known under the name photon scanning tunneling microscope (PSTM), as described for example in the U.S. Pat. No. 5,018,865 is the sample-modulated tunneling of normally internally reflected photons to a sharply pointed optically transparent tip. The source of the photons is the evanescent field produced by the total internal reflection of a light beam from the sample surface.
An internal reflection is caused by placing the sample surface at the hypotenuse face of a total-reflection prism. The light beam enters perpendicular to one of the side faces of the prism to be totally reflected by the hypotenuse face. The spatial variations in the evanescent field intensity form the basis for imaging. They essentially provide an exponentially decaying waveform normal to the sample surface. Photons tunneling from the total internal reflection surface to the tip are guided to a suitable detector which converts the light flux to an electrical signal. The PSTM detects a signal only when the tip is placed within the decay length of the evanescent wave, allowing an accurate distance control.
The main problem is that details having smaller dimension produce evanescent rather than harmonic waves and they do not propagate in space without attenuation. One interesting approach that allows seeing those details is related to the near field tunneling microscope in which a pipette with core which is less than the wavelength is attached to the sample and causes tunneling of the evanescent waves into the pipette [S. I. Bozhevolnyi, B. Vohnsen, E. A. Bozhevolnaya and S. Berntsen, “Self-consistent model for photon scanning tunneling microscopy: implications for image formation and light scattering near a phase-conjugating mirror,” JOSA A, Vol. 13, 2381 (1996)]. After guiding them to the detector they can be observed. Every readout of the detector corresponds to a single spatial sampling point. In order to map the entire sample, scanning is required. In order to have sufficient energy, the pipette must be very close to the surface of the sample, i.e. at a distance which is much smaller than the optical wavelength. The main problem with this approach is that covering large area consumes a lot of time.
Another shortcoming of the PSTM relates to the illumination of the sample: In contrast to the a-SNOM, the whole sample is irradiated throughout the total measuring time. Thus, the probability of damage through heating or other effects of the light is increased. Further, the PSTM shows an inferior lateral resolution compared to a SNOM, due to the use of a transparent optical probe tip. One way of improving this resolution is to cover the tip of a PSTM with an opaque material leaving only a tiny aperture to collect the light from a well-defined spot of the sample. However this is at the cost of the detected light intensity.